Some factors that influence the concentrations and metabolism of plasma low and high density lipoproteins (LDL and HDL) will be studied in squirrel monkeys and rabbits and in isolated rabbit hepatocytes. An understanding of the relationship between lipoproteins and experimental atherosclerosis and cholesterol gallstones is the overall goal. Nutritional factors and sex will be variables, but we will focus particularly on two populations of squirrel monkeys, one of which has high levels of HDL2 and is susceptible to cholesterol gallstones, whereas the other has low levels of HDL2 and no gallstones. We will emphasize the level of lipoproteins and the metobolism of the apoprotein and lipid moieties, as well as the composition and secretion of bile and the turnover of bile acids. We will also seek to verify that HDL cholesterol is the main source of biliary cholesterol and bile acids. Parallel studies of the metabolism of the protein and lipid moieties of HDL and LDL will be continued on isolated rabbit hepatocytes in monolayer.